Vaccinations are a case study of “The Tragedy of the Commons” – where anti-vaxxers become free-riders putting their self-interest over the common good. The Hastings Center explains this problem very well:

“…To understand why, think of vaccination and the quest for herd immunity as a collective action problem. Garrett Hardin’s “tragedy of the commons” illustrates the basic logic of collective action problems. Imagine that 50 farmers share common land (“the commons”) upon which they graze their sheep. The commons are lush, and so each farmer can easily allow four sheep to graze at a given time without depleting the resource. But imagine that each farmer seeks to maximize his own good (what economic theory refers to as “rational” behavior) and it is better for him to graze more sheep than fewer. The farmers will, in effect, be “free-riding” – in this case, taking more than their fair share of the common resource while benefitting from the restraint of others. The trouble is that, while adding one more sheep to the commons does not deplete the resource, adding 50 does. The combined actions of each farmer, acting rationally, leads to an outcome that is worse for all.

The tragedy of the commons reveals that what is good for the individual is at odds with what is good for all. This is the basic logic of collective action problems. We see a similar logic in the case of vaccines. If most get vaccinated, then everyone will be better off. But it would be best for any particular individual if all others got vaccinated and he or she did not. That way, the individual could enjoy the benefits of the common good (herd immunity) without bearing any of the costs (e.g., risk of possible side effects or complications associated with vaccine). This, again, is a free-rider temptation. The trouble is that if everyone thought that way, no one would become vaccinated and everyone would be at risk of falling ill.

From this perspective, anti-vaxxers are not ill-informed parents with distorted views of what is in their child’s best interest. They are acting perfectly rationally. The trouble is that there are enough of them to generate the tragedy of the commons. Hence, vaccination levels drop and measles rates rise. …”